Glazing

ABSTRACT

A laminated glazing panel ( 10 ) has at least one sheet of glass having a thickness of between 0.8 mm and 3.5 mm. The glass sheet has: a) an edge compression stress of between 20 MPa and 80 MPa; and b) a surface compressive stress at a central portion of the glass of between 2 MPa and 39 MPa. The glazing panel may be used as a laminated automotive glazing.

[0001] This invention relates to glazing panels and particularly but notexclusively to laminated glazing panels intended for automotive sidewindows and rear windows.

[0002] Whilst the invention will be described with particular referenceto laminated automotive side windows it may also be used in otherapplications, for example vehicle windscreens, vehicle backlights,vehicle sunroofs, other vehicle glazing panels and other laminatedglazings.

[0003] Car sidelights have traditionally be provided as monolithicthermally tempered glass sheets having a thickness of between about 3 mmand 5 mm. The tempering is arranged to increase the mechanicalresistance of the glass sheets and to ensure that if the glass sheetsshatters it breaks into relatively small, blunt edged pieces so as tominimise potential injury.

[0004] It is also known to provide laminated glass sidelights having twosheets of glass laminated together by means of a sheet of polyvinylbutyral (pvb). It has always been considered that the glass sheets insuch a structure must be tempered or at least hardened so as to induce asignificant level of surface stress in order to provide sufficientmechanical resistance for handling and use and, for example, towithstand repeated slamming of a car door in which the glazing ismounted. The individual sheets of glass in such a structure generallyhave a thickness of less than 3 mm so as to minimise their weight and ithas been difficult or impossible to induce the desired stresses in suchthin glass sheets by many conventional manufacturing techniques.

[0005] Laminated sidelights comprising a single sheet of glass laminatedwith a single sheet of plastics, for example a polyurethane, have alsobeen proposed. In addition to the durability and abrasion considerationsfor such glazings, the difficulty of providing the temperedcharacteristics perceived necessary with relatively thin sheets of glasshas also been problematic.

[0006] According to a first aspect, the present invention provides alaminated glazing panel as defined in claim 1.

[0007] It had previously been thought necessary to have significantlyhigher levels of surface compressive stress for such glazing panels inorder to provide desired mechanical characteristics. It is alsosurprising that relatively low levels of surface compressive stress maybe successfully combined with edge compressive stresses which arerelatively high. The value of the edge compressive stress may be greaterthan the value of the surface compressive stress.

[0008] Particularly in the case of a curved sheet of glass, the definedlevel of surface compressive stress is preferably the stress at 90° i.e.perpendicular to the longitudinal axis of the glass sheet orperpendicular to the main axis about which the glass sheet is curved.The defined level of stress may be present at the concave surface of acurved sheet of glass and/or at the convex surface. The required levelsof surface compressive stress may be satisfied if the majority of thepoints in question fall within the defined levels of stress.

[0009] The edge compressive stresses defined are preferably the maximumstresses at the edges of the glazing panel and may ensure that theglazing is not overly susceptible to breakage at its edges, for exampleduring handling and/or use. The levels of edge compressive stressdefined may be satisfied if the majority of points at the peripheraledge of the glazing fall within the defined limits. Clearly, it is moreimportant that the edges of the glazing panel which are going to beexposed in use, for example the top edge of a vertically moveable cardoor window, conform to the edge stress requirements of the presentinvention rather than edges that will be less susceptible in use eitherto flexion or impact, for example, the lower edge of a verticallysliding car door window which in use is always hidden within the cardoor.

[0010] The surface compressive stress at a central portion of the glassmay be greater than or equal to 5 MPa, 10 MPa or 15 MPa; it ispreferably greater than or equal to 20 MPa.

[0011] Arranging for the edge compressive stress of the glass to have avalue of greater than or equal to about 30 MPa and/or less than or equalto about 65 MPa may provide an advantageous level of edge compressivestress to confer desired mechanical characteristics and/or may beobtainable with certain manufacturing techniques.

[0012] The laminating layer may be a sheet of polyvinalbutyral (PVB)having a thickness of between about 0.2 mm and 1.6 mm. Standard PVBthicknesses for laminated car glazing panels are 0.38 mm and 0.76 mm.The laminating layer may be a composite layer; it may comprises, forexample, two sheets of PVB having another layer, for example a sheet ofpolyethylene terephtalate (PET) sandwiched between them. Such a PET (orother sheet) may carry a solar control coating, for example a magnetronsputtered coating stack adapted to enhance the solar control propertiesof the glazing.

[0013] According to a second aspect, the present invention provides alaminated glazing panel as defined in claim 3.

[0014] It is particularly surprising that glass sheets having edgestresses within the defined range are suitable for the purposesenvisaged.

[0015] The glass sheet may have an edge compression stress of greaterthan about 30 MPa, 40 MPa or 45 MPa.

[0016] The surface compressive stress of the glass sheet at a centralportion of the glass may be greater than 5 MPa, preferably greater than20 MPa; it may be less than 60 MPa or less than 40 MPa.

[0017] Arranging the intermediate portion of the glazing to have atensile stress averaged across the thickness of the glass of betweenabout 0 MPa and 15 MPa may reduce the fragility of this portion of theglazing and reduce its susceptibility to breaking upon impact. In fact,it is almost inevitable for a heat treated glass to have an intermediatezone which averaged across the thickness of the glass is in tension. Theintermediate zone may have a tensile stress averaged across itsthickness of greater than 2 MPa, 4 MPa or 5 MPa and/or less than 10 MPaor 12 MPa.

[0018] The use of one or more glass sheets having a thickness of betweenabout 1.2 mm and 2.5 mm may facilitate reduction in the overall weightof the laminated glazing panel and/or allow a desired overall thicknessof laminated glazing panel to be produced. This may be particularlyuseful for automotive applications. For example, two sheets of glasseach having a thickness of 1.65 mm may be laminated together by a 0.76mm thick layer of pvb to give an overall thickness of 4.06 mm. This maybe interchanged with a monolithic glazing panel having a nominalthickness of 4 mm. It may be possible to use a glazing panel having athickness of up to about 4.4 mm in a sliding arrangement designed for anominal thickness of 4 mm.

[0019] Where the laminated glazing panel has two sheets of glass, eachof these two sheets may have the defined levels of stress. This mayconfer suitable properties on the laminated glazing panel; it may alsofacilitate manufacture by allowing two similar or substantiallyidentical glass sheets to be assembled together to form the, laminatedglazing panel rather than requiring different manufacturing operationfor each of the two glass sheets. In this case, the two glass sheetspreferably have substantially the same thickness; alternatively, the twoglass sheets may have different thicknesses. The two glass sheets may bebent individually, for example by pressing, rather than being bent as apair.

[0020] The present invention may permit the use of certain types ofconventional furnaces and/or manufacturing techniques that could notreadily be used for prior art glazings.

[0021] The laminated glazing panel may be curved or substantiallynon-planar. In this case, the desired stresses are preferably induced aspart of an operation or process by which the glass is bent or shaped.

[0022] Preferably, the surface stress characteristics are substantiallysimilar over the entire surface of the glass, or over the majority ofthe surface of the glass or over the majority of the central portion ofthe glass. This may confer substantially similar breakingcharacteristics over all of or at least over the majority of the surfaceof the sheet of glass. Clearly, the breaking characteristic at any edgeportion and at any intermediate portion are unlikely to be identical tothat at the central portion. Nevertheless, the surface stresscharacteristics may be substantially uniform in the sense that the glasspreferably does not have differentiated stress zones.

[0023] The glass of the glazing panel is preferably a soda-lime glass,more preferably float glass. It may comprise the following constituent(expressed in percentage by weight): SiO₂ 60 to 75% Na₂O 10 to 20% CaO 0to 16% K₂O 0 to 10% MgO 0 to 10% Al₂O₃ 0 to 5% BaO 0 to 2% BaO + CaO +MgO 10 to 20% K₂O + Na₂O 10 to 20%

[0024] According to further aspects, the present invention provides alaminated glazing as defined in claim 9 and for a method ofmanufacturing a laminated glazing as defined in claim 11.

[0025] An embodiment of the present invention will now be described, byway of example only, with reference to the accompanying drawings ofwhich:

[0026]FIG. 1 is a plan view of a laminated glazing panel; and

[0027]FIG. 2 is a cross section through line 2-2 of FIG. 1.

[0028] For ease of representation, the figures are not drawn to scale.

[0029] The accompanying drawings show a laminated glazing panel whichforms a moveable side window of a car. The glazing 10 comprises first 11and second 12 sheets of glass laminated together by means of asubstantially transparent pvb laminating layer. It may be manufacturedby cutting the desired peripheral shape of each sheet of glass from aflat, planar glass sheet to provide a blank, passing the blank through afurnace to raise its temperature sufficiently to allow it to be bent,pressing the heated blank against a convex mould to curve the blank,allowing the curved blank to cool and subsequently assembling andlaminating it with a second similarly produced blank to produce alaminated glazing panel. Alternatively, a method of bending by gravityrather than by pressing may be used.

[0030] In the illustrated example, each of the glass sheets 11,12 has athickness of 1.7 mm and has a similar stress distribution.

[0031] As indicated by the dotted lines in FIG. 1, the glass sheet maybe considered to have three distinct stress zone, being:

[0032] a) a central zone 21 at which the core of the glass sheet issubject to tensile stresses and at which the surfaces of the glass sheetare subjected to compressive stresses and at which the total stressaveraged across the thickness of the glazing is substantially zero;

[0033] b) an intermediate zone 22 at which the core of the glass sheetis subject to tensile stresses and at which the surfaces of the glasssheet are subjected to compressive stresses and at which the totalstress averaged across the thickness of the glazing is a tensile stress;and

[0034] c) an edge region 23 at which the total stresses averaged acrossthickness of the glass sheet are compressive stresses.

[0035] The exact boundary between the different stress zones may not beclearly discernible.

[0036] In general, the edge region will have a width of between abouttwo to three times the thickness of the glass from the peripheral edgeof the glass sheet and the intermediate region will have a width of upto about ten times the thickness of the glass.

[0037] Table 1 sets out the surface stress at three point in the centralregion at the convex side of one of the sheets of glass: TABLE 1Position Surface compression at 90° 31 24.2 MPa 32 22.8 MPa 33 22.3 MPa

[0038] The surface compression is measured using a Gasp measuring devicefrom Strainoptic Inc. which is placed so as to direct its laser beamalong the axis at 0° and consequently measure the surface compression at90°. Where the glass is a float glass, it is preferable to measure thesurface compressive stress of the tin face, i.e. the face of the glassthat has been in contact with the tin float bath during its manufacture.

[0039] The surface compression at the central zone 21 of the glass maybe non-homogeneous i.e. it may vary as a function of the direction inwhich it is measured.

[0040] Table 2 sets out the maximum tensile stress averaged across thethickness of the glass at positions of the intermediate regioncorresponding to the positions indicated at the periphery of the glazingpanel: TABLE 2 Position Tensile stress 41 1.1 MPa 42 4.4 MPa 43 7.8 MPa44 5.6 MPa 45 1.1 MPa 46 3.3 MPa 47 2.1 MPa 48 4.2 MPa 49 7.1 MPa 50 7.8MPa 51 5.6 MPa 52 3.3 MPa

[0041] The tensile stress is measured using an optical measurementtechnique with the help of a Babinet comparator.

[0042] Table 3 sets out the maximum compressive stress measured attwelve points of the edge region around the periphery of the glazingpanel: TABLE 3 Position Compressive edge stress 41 45.2 MPa 42 44.0 MPa43 42.7 MPa 44 44.0 MPa 45 47.6 MPa 46 51.3 MPa 47 48.8 MPa 48 41.5 MPa49 47.6 MPa 50 48.8 MPa 51 50.1 MPa 52 42.7 MPa

[0043] The compressive edge stress is measured using an opticalmeasurement technique with the help of a Babinet comparator.

[0044] Preferably, glazings according to the invention conform toEuropean standard R43.

[0045] The present invention may permit a single furnace to be used tomanufacture both monolithic tempered glazing panels and laminatedglazing panels in accordance with the invention. Suitable adjustmentsmay be made to the furnace conditions and to the manufacturing processto enable this.

1. A laminated glazing panel comprising at least one sheet of glasshaving a thickness of between 0.8 mm and 3.5 mm characterised in thatglass sheet has: a) an edge compression stress of between 20 MPa and 80MPa; and b) a surface compressive stress at a central portion of theglass of between 2 MPa and 39 MPa.
 2. A laminated glazing panel inaccordance with claim 1, in which the glass sheet has an edgecompression stress of between 30 MPa and 65 MPa.
 3. A laminated glazingpanel comprising at least one sheet of glass having a thickness ofbetween 0.8 mm and 3.5 mm characterised in that glass sheet has: a) anedge compression stress of between 20 MPa and 49 MPa; and b) a surfacecompressive stress at a central portion of the glass of between 2 MPaand 80 MPa.
 4. A laminated glazing panel in accordance with anypreceding claim, in which at an intermediate portion of the glass sheetpositioned between the central portion and the portion having thedefined edge compressive stress, the glass sheet has a tensile stressaveraged across the thickness of the glass sheet of more than 0 MPa andless than 15 MPa.
 5. A laminated glazing panel in accordance with anypreceding claim, in which the glass sheet has a thickness of between 1.1mm and 2.6 mm.
 6. A laminated glazing panel in accordance with anypreceding claim, in which the glazing panel comprises two sheets ofglass laminated together by means of a substantially transparentinterlayer.
 7. A laminated glazing panel in accordance with anypreceding claim, in which the glazing panel is an automotive side windowor an automotive rear window.
 8. A glass sheet adapted to be assembledas a laminated glazing panel in accordance with any one or more of thepreceding claims in which the glass sheet has the characteristics of theglass sheet defined in any one or more of the preceding claims.
 9. Alaminated glazing panel comprising two glass sheets in accordance withclaim
 8. 10. A laminated automotive side window or rear window inaccordance with claim
 9. 11. A method of manufacturing a laminatedglazing panel in accordance with any preceding claim which comprises twosheets of glass in which each sheet of glass is bent individually toconfer upon it a desired curvature and in which the two sheets aresubsequently assembled together and laminated together to form thelaminated glazing panel.